1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, in particular, to a control apparatus for an internal combustion engine, which detects and suppresses abnormal combustion due to a knock, pre-ignition, and post-ignition, which occurs in the internal combustion engine.
2. Description of the Related Art
A method of detecting a knock, which corresponds to a kind of abnormal combustion occurring in an engine, by a vibration sensor is conventionally known. It is known that, if the knock occurs while the engine is in operation, vibrations in a specific frequency band occur according to a vibration mode of the engine or that of the knock. Therefore, the conventional knock detection is performed by measuring a vibration intensity of the specific frequency.
Specifically, an analog bandpass filter circuit is used to extract the specific frequency. A method of measuring the vibration intensity with a bandpass-filtered peak-hold value, which is obtained by inputting an output of the bandpass filter circuit to a peak-hold circuit, has been proposed (for example, see JP 2002-357156 A). A method of performing digital signal processing (for example, fast Fourier transform: FFT) to measure the vibration intensity with a spectral value of the specific frequency obtained by the digital signal processing has also been proposed (for example, see JP 3093467 B).
Besides the knock, pre-ignition and post-ignition are also known as the abnormal combustion occurring in the engine. The abnormal combustion due to the knock is as follows. First, after spark ignition with a spark plug, a flame propagates from the spark plug. At this time, an unburned air-fuel mixture (end gas), which is at a location distant from the spark plug, is pressed against a piston or a wall surface of a cylinder to be placed in a high-temperature and high-pressure state. As a result, when the end gas is self-ignited, a shock wave is generated. It is believed that the generation of the shock wave causes an excessively high in-cylinder pressure, a vibration of an engine block, and a metallic sound in the engine block.
The following two cases are considered as abnormal ignition called the pre-ignition or the post-ignition, for example. In the first case, the spark plug or a deposit present in the cylinder has an elevated temperature to be a heat source, leading to ignition (hereinafter, referred to as “heat-source self-ignition”). In the second case, when a compression ratio is high, a temperature and a pressure of the air-fuel mixture become high during a compression stroke to cause the self-ignition (hereinafter, referred to as “compression self-ignition”). Even in such a case, the excessively high in-cylinder pressure, the vibration of the engine block, and the metallic sound in the engine block are sometimes caused.
The abnormal ignition which is caused prior to normal spark ignition, is called the pre-ignition, whereas the abnormal ignition which is caused after the normal spark ignition, is called the post-ignition. The above-mentioned kinds of abnormal combustion are generally known (for example, see “University Lecture; Internal Combustion Engine (original title: Daigakukogi; Ninenkikan)” by Itsuro Kimura and Tadami Sakai, Maruzen, 1980, pp. 82 to 84, and “Lecture on Internal Combustion Engine (original title: Nainenkikan Kogi)”, Vol. 1, by Fujio Nagao, 1980, pp. 216 to 223). If any one of the kinds of abnormal ignition occurs, the generation of an uncomfortable metallic sound or a fluctuation in engine output occurs. In an extreme case, the engine is damaged.
Even when the pre-ignition or the post-ignition (hereinafter, collectively referred to as the abnormal ignition) occurs, the vibrations of the engine sometimes occur at the same time. Therefore, a method of detecting the abnormal ignition by using the above-mentioned knock detection method has been proposed. As such a method, a method of determining the occurrence of the abnormal ignition based on a difference in time between ignition timing and a time of occurrence of the vibration is known (for example, see JP 3082634 B). Moreover, the following method of judging that the abnormal ignition has occurred is known. According to this method, when the vibrations due to the knock are not stopped within a predetermined period of time even by correcting the ignition timing to a retard side to suppress the knock after the detection of the knock, the vibrations due to the knock increase, or the vibrations due to the knock occur even after the ignition timing is retarded to most-retarded ignition timing, it is judged that the abnormal ignition has occurred (for example, see JP 3082634 B, JP 11-93757 A, and JP 11-247750 A).
As a method of suppressing the knock, a method of retarding the ignition timing is generally known (for example, see JP 2002-357156 A). In the case of the engine with a high compression ratio, it is known that a method of reducing an effective compression ratio after the ignition timing is retarded to the most-retarded position is also effective (for example, see JP 3937680 B). Further, as a method of suppressing the abnormal ignition, a method of reducing the amount of intake air, a method of increasing the amount of fuel injection (for example, see JP 3082634 B and JP 61-187558 A), and a method of fuel cut (for example, see JP 11-247750 A) are known.
However, the prior art has the following problems.
According to the conventional judgment method based on the difference in time between the ignition timing and the time of occurrence of the vibration, it is difficult to specify the time of occurrence of the vibration due to the abnormal ignition because the engine is always vibrated during the operation. Further, even if a position of a rise or peak of a relatively large vibration is detected as the time of occurrence of the vibration, each of the position of the rise and the position of the peak varies considerably. Therefore, the normal knock is frequently erroneously judged as the abnormal ignition.
Moreover, in the conventional judgment method of judging that the abnormal ignition has occurred when the vibrations due to the knock are not stopped within the predetermined period of time even after the ignition timing is corrected to the retard side, the vibrations due to the knock increase, or the vibrations due to the knock occur even after the ignition timing is retarded to the most-retarded ignition timing, there is a problem in that a torque is greatly lowered because the ignition timing is continuously retarded. Further, the judgment is made based on the continuous occurrence of the vibrations due to the knock over the predetermined period of time, and hence the above-mentioned conventional judgment method is also disadvantageous in that the uncomfortable vibrations or metallic sounds are continuously generated until the judgment of the abnormal ignition is made.
The above-mentioned problems are described further in detail with reference to FIGS. 9, 10A, and 10B. FIG. 9 is an explanatory view illustrating a relation between the ignition timing, the effective compression ratio, and an abnormal combustion area, which is obtained by an experiment conducted on the compression self-ignition. FIGS. 10A and 10B are explanatory views illustrating relations between the ignition timing and the torque, and between the effective compression ratio and the torque, respectively. The problem here is the case where a combustion state cannot get out of the abnormal combustion area even when the ignition timing is continuously retarded, as indicated by a dotted-line arrow (b′) illustrated in FIG. 9. If the ignition timing is further continuously retarded in this case, the torque is greatly lowered, as illustrated in FIG. 10A.
Even with the conventional knock suppressing method of lowering the effective compression ratio after retarding the ignition timing to the most-retarded position in the engine with the high compression ratio, there is a problem of a remarkable reduction of the torque because the ignition timing is first retarded to the most-retarded position, as indicated by the dotted-line arrow (b′) illustrated in FIG. 9 and as illustrated in FIGS. 10A and 10B. Moreover, as illustrated in FIG. 9, there is a possibility that the abnormal combustion occurring in the area is not due to the knock but due to the post-ignition or the pre-ignition. Therefore, there is another problem in that the abnormal combustion cannot be sufficiently suppressed only by reducing the effective compression ratio.